Create example(s) showing how to bind to Subxt from other languages

[jsdw]

Create an example showing how to bind to Subxt from several languages (eg, Python, Java) and use it to construct and/or submit transactions.

We should decide which language(s) are most important for the community and then create example bindings to those to accomplish a simple task (like submitting a transaction).

A single example should be applicable to several languages: we'd create bindings once and then show how they can be used in target langauges. We don't want to create bindings multiple times as this will be more of a maintenance headache.